Induction of the cytochrome P450 3A subfamily in rat liver correlates with the binding of inducers to a microsomal protein.

The specific binding of the archetypal cytochrome P450 3A subfamily (CYP3A) inducer dexamethasone is examined in microsomal fractions since rat and human liver CYP3A is induced by glucocorticoids through a mechanism which is apparently independent of the cytosolic glucocorticoid receptor. Dexamethasone binds in a specific and saturable manner to microsomes with an affinity constant (Kd) of 59 +/- 12.9 nM which compares to a Kd of 2.3 +/- 0.17 nM in cytosol. The total receptor concentrations ([LR]emax) in microsomes and cytosol are 9.5 +/- 1.67 pmols/mg protein and 410 +/- 167 fmol/mg protein respectively. The microsomal binding of dexamethasone is antagonised by several transcriptional and/or post-transcriptional CYP3A inducers with decreasing potency pregnenolone 16 alpha carbonitrile > metyrapone > phenobarbitone. Troleandomycin, which indirectly induces CYP3A1 in vivo and by protein stabilisation, does not antagonise the binding of dexamethasone in microsomes. The transcriptional and/or post-transcriptional induction of CYP3A may therefore be associated with the interaction of inducers with a microsomal protein.